Saturn's moon Phoebe might be a planetesimal—a remnant of the rocky building blocks of the planets in our Solar System. A new study by Julie C. Castillo-Rogez et al. from Cassini spacecraft data indicates that Phoebe dates back to the very earliest days of the Solar System. Based on surface features and evidence that the moon is significantly more dense than the larger Saturnian satellites, the astronomers argue that Phoebe likely formed much farther from the Sun then fell inward, where it was snagged by Saturn's gravity.

Using detailed observations from Cassini and Earth-based telescopes, in combination with detailed computer simulations, Castillo-Rogez et al. determined that Phoebe began as a spherical body. Based upon the density and comparison with bodies of similar size, Phoebe may have a rocky core surrounded by a porous icy shell. The layered structure grants Phoebe kinship with the planets and the planet-like asteroid Vesta, as well as the larger Kuiper Belt objects such as Pluto. Phoebe's physical properties, as well as its odd orbital characteristics, led the authors to conclude that the moon formed in the Kuiper Belt region, making it a cousin to Pluto.

Phoebe's orbit is retrograde, meaning it circles in the opposite direction to Saturn's other moons, as well as the planet's rotation. It's also significantly non-circular. These orbital characteristics are in contrast to the so-called regular satellites: those with orbits the same direction as the planet's rotation and largely circular shape. Regular satellites likely formed along with Saturn, while irregular satellites were captured after formation, according to standard astronomical models.

While Phoebe is obviously not a smooth sphere, it's also not as potato-like as many other moons of similar size. Castillo-Rogez and her collaborators showed that its shape is consistent with an oblate spheroid (meaning squashed at the poles and bulging at the equator, like Earth). Comparing the global shape and surface topology—craters, ridges, and so forth—they concluded that Phoebe began as as a spherical object, but battering by collisions during its early life changed its shape, creating the lumpier body we see today.

Additionally, the researchers modeled the interior, using the moon's density and shape. They determined that Phoebe likely has a differentiated interior, meaning that different materials have formed separate layers. The models suggest that Phoebe has a rocky core surrounded by a shell of water ice.

Many other moons, such as Enceladus have a greater water content in their core, while objects like comets and most asteroids are completely undifferentiated. In this sense, Phoebe more closely resembles planets, Pluto, and the asteroid Vesta in that regard. An object structured like this also is likely to have formed in the early Solar System, since later bodies should not have become layered. This places Phoebe's formation no later than 3 million years after the beginning of the Solar System.

Based on its higher density and probable interior composition, Phoebe resembles objects in the Kuiper Belt, the region beyond Neptune's orbit where Pluto, Eris, and many comets reside. Its density is 1.63 times that of water, noticeably greater than the average for the inner Saturnian satellites (including Titan, Enceladus, and so forth), which is 1.24 times water's density. Pluto's density, by comparison, is approximately twice that of water.

Phoebe may have started out with Pluto's density, but the heavier bombardment it experienced may have disrupted it, stripping away some material and turning the outer layers into low-density fragments. Similar effects are seen in other bodies, to the extent where some have become piles of rubble held together by gravity; Phoebe occupies a middle ground between rubble piles and larger cratered spherical objects.

Finally, Phoebe's surface composition includes a lot of water ice mixed with carbon compounds, so it somewhat resembles the "dirty snowball" comets. While any of these details by themselves don't indicate a Kuiper Belt origin (for example, the carbon could have been deposited after formation), all together the picture suggests that Phoebe did originate beyond Neptune's orbit. The combination of surface characteristics (composition and cratering), shape, high density and probable differentiation in the interior, and orbital properties argue persuasively that Phoebe is a planetesimal that formed in the Kuiper Belt, more akin to Pluto than to Titan.